I think i know why: I'm an idiot. So of course just the one pipe is the inlet otherwise the coolant would flow to various places and not directly into the heater core.

I"m going with that. Unless anyone corrects me Thanks.

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius